Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
www.acsami.org
Individual Nanoporous Carbon Spheres with High Nitrogen Content from Polyacrylonitrile Nanoparticles with Sacrificial Protective Layers Jianan Zhang,†,‡,§,# Rui Yuan,†,# Sittichai Natesakhawat,∥,⊥ Zongyu Wang,† Yepin Zhao,‡ Jiajun Yan,† Siyuan Liu,‡ Jaejun Lee,‡ Danli Luo,‡ Eric Gottlieb,† Tomasz Kowalewski,† Michael R. Bockstaller,*,‡ and Krzysztof Matyjaszewski*,† †
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States § School of Chemistry and Chemical Engineering, Anhui University and Anhui Province Key Laboratory of Environment-Friendly Polymer Materials, Hefei 230601, China ∥ National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States ⊥ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡
S Supporting Information *
ABSTRACT: Functional nanoporous carbon spheres (NPCS) are important for applications ranging from adsorption, catalysis, separation to energy storage, and biomedicine. The development of effective NPC-S materials has been hindered by the fusion of particles during the pyrolytic process that results in agglomerated materials with reduced activity. Herein, we present a process that enables the scalable synthesis of dispersed NPC-S materials by coating sacrificial protective layers around polyacrylonitrile nanoparticles (PAN NPs) to prevent interparticle cross-linking during carbonization. In a first step, PAN NPs are synthesized using miniemulsion polymerization, followed by grafting of 3-(triethoxysilyl)propyl methacrylate (TESPMA) to form well-defined core−shell structured PAN@PTESPMA nanospheres. The cross-linked PTESPMA brush layer suppresses cross-linking reactions during carbonization. Uniform NPC-S exhibiting diameters of ∼100 nm, with relatively high accessible surface area (∼424 m2/g), and high nitrogen content (14.8 wt %) was obtained. When compared to a regular nanoporous carbon monolith (NPC-M), the nitrogen-doped NPC-S demonstrated better performance for CO2 capture with a higher CO2/N2 selectivity, an increased efficiency in catalytic oxygen reduction reactions, as well as improved electrochemical capacitive behavior. This miniemulsion polymerization-based strategy for the preparation of functional PAN NPs provides a new, facile approach to prepare highperformance porous carbon spheres for diverse applications. KEYWORDS: nanoporous carbon, polyacrylonitrile, carbon, miniemulsion polymerization, core−shell nanoparticle, SI-ATRP
■
INTRODUCTION
air purification, biomedicine, and energy storage and conversion.21 However, such nanoporous carbons are hydrophobic and have a limited number of active sites which impede their broader commercial applications.22 Doping of NPC-S with heteroatoms such as nitrogen, sulfur, oxygen, and fluorine resulted in significantly enhanced surface polarity, electrical conductivity, and electron-donor tendency of the mesoporous carbons.23,24 This enhanced NPC-S performance in CO2 capture, electric double-layer capacitors (EDLCs), fuel cells, and catalysis.25,26 Further improvements of NPC-S require
Nanoporous carbons (NPC) are attracting significant scientific and technological interest due to their potential applications in many areas including catalysis,1,2 water and air purification,3,4 and energy storage and conversion.5,6 Various synthetic strategies have been developed to prepare NPC, including direct carbonization of cross-linked/conjugated polymers and metal−organic frameworks (MOFs),7,8 high-temperature chlorination of carbide materials,9,10 self-assembly of block copolymer templates,11−17 and hard templated nanoporous silica and zeolites.18−20 Recently, nanoporous carbon spheres (NPC-S) possessing unique properties of controllable porous structure and spherical morphology find applications in catalysis, adsorption, water and © XXXX American Chemical Society
Received: August 9, 2017 Accepted: October 10, 2017
A
DOI: 10.1021/acsami.7b11910 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Process to Synthesize NPC-S and NPC-Ma
a
(a) Formation of PAN NPs by miniemulsion polymerization. (b) Preparation of PAN NP-g-PTESPMA brushes from PAN NP cores by SI-ATRP. (c) ZnCl2 impregnation, thermal stabilization, and carbonization of PAN@PTESPMA under a nitrogen atmosphere to stabilize the structure of the nanostructured carbon/silica hybrids. (d) HF etching to generate NPC-S. (a*) Preparation of NPC-M by direct carbonization of PAN NPs without a protective shell. 99%, Aldrich), N,N-dimethylformamide (DMF, Fisher), carbon tetrabromide (CBr4, 99%, Aldrich), and hydrofluoric acid (HF, 50 wt %, Acros) were used as received. Synthesis of Functional PAN NPs by Miniemulsion Polymerization. Cross-linked PAN NPs were prepared via miniemulsion polymerization at 65 °C. In a typical synthesis, an oil phase was prepared by mixing HD (0.8 g) and AIBN (0.3 g) with a mixture of AN (15.0 g) and DVB (1.8 g) by mechanical stirring for 0.5 h. A portion of SDS aqueous solution was employed as the water phase. The miniemulsion was prepared by pre-emulsification of the mixture of oil phase and water phase by mechanical stirring for 0.5 h and subsequent sonication for 10 min in an ice bath. The polymerization was carried out at 65 °C. After 30 min of polymerization at 65 °C, when the conversion was nearly 70%, a CBr4 solution (0.01 g of CBr4 in 1.0 g of monomer) was added to the ongoing miniemulsion polymerization.37 The polymerization was allowed to proceed for 8 h under N2 atmosphere to obtain the cross-linked PAN NPs with accessible ATRP initiating sites on each particle. The obtained PAN NPs were used for the preparation of PAN-g-PTESPMA brushes after the purification and drying process.37 The yield of PAN NPs was 91 wt % by gravimetric analysis. Preparation of N-Doped NPC-S from PAN-g-PTESPMA Brushes. In a typical synthesis, cross-linked PAN NP-g-PTESPMA brushes were prepared by grafting of TESPMA via SI-ATRP.37 A certain amount of purified particle brushes were added to 10 mL of an aqueous ZnCl2 solution, and the concentration of ZnCl2 in the aqueous solution was adjusted to 60 wt % by adding water. The mixture was stirred for 24 h at room temperature, and then the resulting PAN/silica solution was freeze-dried to obtain solid PAN/ silica core/shell composite NPs. Silica−carbon composites were obtained after carbonization at 700 °C for 2 h under nitrogen. Individual N-doped nanoporous carbon spheres (NPC-S) were generated after complete removal of the silica shell by stirring in 50 wt % aqueous HF solution for 12 h. For comparison, nanoporous carbon monolith (NPC-M) was obtained under the same procedure using pure PAN NPs as the precursor. Characterization. The resulting nanocarbon materials were studied by transmission electron microscopy (TEM) operating at 200 kV. Nitrogen adsorption isotherms were measured in a Micromeritics ASAP 2010 volumetric gas adsorption analyzer. X-ray diffraction (XRD) patterns were recorded on a Rigaku Geigerflex equipped with a theta/theta goniometer. The Raman spectra were collected using a Jobin Yvon T64000 triple Raman system (ISA, Edison, NJ) in subtractive mode with microprobe sampling optics. Excitation was at 514.5 nm (Art laser, Model 95, Lexel Laser, Fremont,
synthesis of nitrogen-doped NPC-S with both high nanoporosity and unique catalytic properties within the carbon frameworks. Indeed, over the past decade, there has been explosive growth in the synthesis, characterization, and application of NPC-S materials. Synthetic strategies include templating,27 hydrothermal carbonization (HTC),28,29 emulsion polymerization,30 self-assembly,31 and modified Stöber methods.32 Because of chain cross-linking and fusion between neighboring nanospheres during high-temperature carbonization, NPC-S materials often agglomerate, diminishing their performance. Thus, the synthesis of uniform “individual” NPC-S presents an outstanding challenge of significant technological relevance.33 Herein, we demonstrate a novel core/shell particle-based strategy for the preparation of uniform NPC-S with high surface area and high nitrogen content. This method is based on the miniemulsion synthesis of functional polyacrylonitrile (PAN) nanoparticles (NPs) and the subsequent grafting of poly(3-(triethoxysilyl)propyl methacrylate (PTESPMA) from the particles to form PAN@PTESPMA core/shell particles through surface-initiated atom transfer radical polymerization (SI-ATRP).34−36 The cross-linked PTESPMA layer acts as a protective shell and sacrificial barrier to avoid fusion of neighboring PAN cores during the carbonization process, thereby generating individual NPC-S after removal of the outer shell. The prepared N-doped NPC-S are effective catalysts in multiple applications. The presented easily scalable process forms NPC-S with the desired nanostructure and functionality, as confirmed by enhanced performance in numerous applications.
■
EXPERIMENTAL SECTION
Chemicals. Acrylonitrile (AN, 99.9%, Acros), divinylbenzene (DVB, 80%, Aldrich), and (3-(triethoxysilyl)propyl methacrylate (TESPMA, Acros) were purified directly before use to remove the stabilizer. Copper(I) chloride (99.999%, Aldrich) was purified by washing with acetic acid several times followed by filtration and stored over nitrogen before use. 2,2′-Azobis(2-methylpropionitrile) (AIBN, 98%, Aldrich) was purified by recrystallization from ethanol. Copper(II) chloride (99.999%, Aldrich), sodium dodecyl sulfate (SDS, Aldrich), 2,2-bipyridine (99%, Aldrich), hexadecane (HD, ≥ B
DOI: 10.1021/acsami.7b11910 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces CA). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi X-ray photoelectron spectrometer microprobe, with a 900 μm spot size. The size and size distribution of the obtained samples were investigated by a dynamic light scattering (DLS) particle size analyzer (Malvern Zetasizer Nano ZS). Gas Sorption Measurements. Sorption isotherms of CO2 and N2 were collected using a Quantachrome Autosorb 1-C. Prior to measurements, samples were degassed at 300 °C under vacuum overnight. Brunauer−Emmett−Teller (BET) surface areas were determined from N2 adsorption isotherms at 77 K. Multipoint BET measurements were performed at relative pressures (P/P0) in the range of 0.05−0.2 CO2, and N2 adsorption isotherms were conducted in the temperature and pressure ranges of 0−50 °C (only 25 °C for N2) and 0−760 Torr, respectively. The pore size distribution (PSD) was obtained from Barett−Joyner−Halenda (BJH) method from the adsorption branch. Electrochemical Measurements. The electrode for oxygen reduction reaction (ORR) was performed according to our previously reported process.14 Potentials were recorded versus an Ag/AgCl reference electrode. All potentials were converted to the reversible hydrogen electrode (RHE) according to the equation: E(RHE) = E0(Ag/AgCl) + E(Ag/AgCl) + 0.0059 × pH. Kinetics of the ORR process was followed by Koutecky−Levich analysis of linear sweep voltammograms using Koutecky−Levich equations. The number of electrons transferred (ne) and the kinetic-limiting current jK can be obtained from the slope and intercept of the Koutecky−Levich plots (1/j versus ω−1/2), respectively. Fabrication of Supercapacitor Devices. Electrochemical measurements were carried out on a potentiostat (Biologic SP-300) in a two-electrode system with a 1 M aqueous Na2SO4 electrolyte. Electrochemical cells were prepared using a Teflon Swagelok with porous polyolefin separator (Celgard Inc.). The cyclic voltammetry (CV) curves were obtained at various scan rates from 2 to 2000 mV/s in the range of 0 to 0.8 V. Galvanostatic charge−discharge (GCD) curves were obtained at various current densities from 0.1 to 20 A/g. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 0.1 Hz to 800 kHz with a sinusoidal signal of 5 mV. The electrodes were fabricated by mixing the 70 wt % of NPC-S, 15 wt % acetylene black as a conductive additive, and 15 wt % of poly(vinylidene fluoride) as a binder. The mixture was homogenized in NMP by sonication for 10 min, and the slurry was deposited on a nickel foam with a diameter of 13 mm and dried in a 100 °C vacuum oven for 24 h.
However, without a protective shell, NPC-M was obtained by direct carbonization of PAN NPs (Scheme 1a*). Morphology of PAN NPs and NPC-S. Transmission electron microscopy (TEM) images of PAN NPs, core−shell structured PAN@PTESPMA nanospheres, NPC-S templated from PAN@PTESPMA nanospheres, and NPC-M prepared from cross-linked PAN NPs without the grafted PTESPMA are shown in Figure 1. Bromo-initiators for ATRP were
Figure 1. TEM images of (a) cross-linked PAN NPs, (b) core−shell structured PAN@PTESPMA nanospheres prepared by SI-ATRP, (c) NPC-S templated from PAN@PTESPMA nanospheres after the removal of the protective shell, and (d) NPC-M prepared from crosslinked PAN NPs without the grafted PTESPMA. Scale bars are 100 nm in all figures.
immobilized on the surfaces of cross-linked PAN NPs by miniemulsion polymerization using CBr4 as the chain transfer agent.37 The as-prepared PAN NPs had quite a uniform size with a diameter
